A refrigerator cabinet comprises three essential structural components. The first is the outer shell which is of metal and is comparatively thin but tough enough to withstand normal kitchen wear and tear. The second is a molded plastic inner layer providing a hygienic interior surface able to withstand a degree of rough treatment at quite low temperatures.
The plastic inner layer is conventionally an impact resistant styrenic polymer such as rubber modified polystyrene (HIPS) or the product of polymerizing styrene and acrylonitrile in the presence of a rubber such as polybutadiene (ABS). Alternatively, the plastic inner layer can be formed of any other suitable tough polymer, for example a polyester such as polyethylene terephthalate, a polycarbonate, a polyacetal or a blend of polyphenylene oxide with HIPS such as "Noryl" (Trade name of General Electric Company). In general however, ABS is by far the most widely used polymer for producing refrigerator inner layers and the description of the invention that follows is couched in terms of ABS. It should be understood however, that this is a matter of convenience only and implies no necessary limitation on the scope of the invention.
Intermediate these layers is the third component. This is a foam material and provides the insulation between the interior of the cabinet and the ambient air.
The foam is produced in situ and is usually a conventional rigid polyurethane foam formed by reaction of an isocyanate with, for example, a polyol or polyamine. It is not essential that the foam be a polyurethane but as a matter of practical convenience and economy it usually is. The blowing agent is usually a halohydrocarbon though other materials that are gaseous or produce gases under polymer formation conditions, can be used.
In a conventional refrigerator the inner and outer layer are located in position and a foam is produced in situ so as to fill the intervening space and hold the inner layer in its appointed position.
It is found however, that in the normal use of the refrigerator a temperature gradient is set up across the thickness of the wall and, if the foam is tightly bonded to both the outer metal shell and the plastic inner layer the foam transmits the stresses induced by differential expansion and contraction of the metal and foam layers to the inner layer and this can often, in time, give rise to stress cracking of the inner layer.
In an attempt to prevent this it has been proposed in U.S. Pat. No. 3,923,355 to coat the foam-contacting surface of either the outer or inner layer with an elastomeric adhesive permitting limited movement between the foam and the layer. An alternative but expensive approach is to coat the foam contacting surface of the inner or outer layer with a release agent only in those areas susceptible to stress concentrations.
A further solution proposed in U.S. Pat. No. 3,960,631 is to interpose between the foam and the plastic inner layer a release film capable of forming a weak bond with the foam that may be broken by the forces engendered by differential expansion of the layers. However, in the event that an inner layer is damaged and has to be discarded it has been found necessary to perform the laborious and time-consuming task of manually stripping the liner film from the inner layer, to which it adheres well, before the inner layer can be reground and re-formed. This is because admixture of the films conventionally used to provide the release layer has a severely deleterious effect on the impact strength of the polymers from which the inner layer is conventionally made.
The present invention represents an improvement in the approach described in U.S. Pat. No. 3,960,631 in that it obviates the need for the stripping of the release film from the inner layer and so substantially reduces the expense of re-processing scrap inner layers.